The rotating blades of the main rotor of a helicopter are subjected to a vibratory load as blades pass through the air. This vibratory load is passed along the blade at the blade passage frequency to the main rotor hub. These vibratory forces are transferred to the rotor mast in three directions; vertically along the axis of the rotor mast, longitudinally (in-plane) in the direction of flight, and longitudinally (in-plane) in a direction perpendicular to the direction of flight. By installing a vibration absorber at the hub, one can minimize excitation inherent to transverse airflow into a rotating wing virtually at the source.
Prior bifilar devices have the disadvantage that a large portion of the installed weight is ineffective. In addition, such devices rely on sliding or rolling metal surfaces in high, centrifugal fields which are not desirable from the reliability and maintainability point of view.
Another system which has been used is a weight installed on top of the rotor head by attaching the weight to a ball joint installed in the mast. Springs center the mass and allow the weight to flap in the horizontal plane. These devices rely on the ball joint to provide vertical retention which blocks off the hollow rotor mast. A large portion of the installed weight is not available for vibration reduction. A similar design employs spring arms with the inner ends bolted to the mast and the outer ends to the weight. This is a complex system that also does not use a significant portion of the weight effectively.
In another device, a group of air filled bladders and mechanical springs have to be used to support a mass. The pressure in the bladders can be varied to tune the device. While such devices have been described in the literature, they have not been employed commercially.
In order to be more effective, a vibration absorption system should react in all five directions (in-plane front and side, vertical, pitch and roll). The present invention in one aspect provides a system where the weight effectively compensates for vibration in all five directions. The present invention also has as an advantage that it can be constructed such that it has an opening along the axis of the rotor mast. Such an opening has the advantage of permitting electrical wiring and other conduits to pass through the inside of the rotor mast, above the rotor blades and beyond the vibration absorber. This allows for the easy connection of above the rotor equipment such as a mast mounted sight and slip ring. In another aspect, the present invention has the advantage of being designed to minimize vibration in an individual helicopter rotor system by varying the mass of the vibration absorber. In another aspect, the present invention relates to use of a motion limiter which allows a smaller mass to be used more efficiently in reducing vibration than conventional vibration absorbers. The system in another aspect is simple to maintain and simple to inspect. The absorber also has the advantage that multiple load paths can be provided to make the system redundant.